For most of the past 20 years, gay Americans dreaded Fred Phelps Sr. and the “God hates fags” pickets by members of his Westboro Baptist Church, a congregation that believes that disciples who do not attack those they see as sinful will themselves be punished by God.

But amid reports that Mr. Phelps is now seriously ill at a Topeka, Kan., hospice facility, some religion experts and gay activists are suggesting that the gay rights movement may ultimately owe Phelps a debt of gratitude for his leadership of one of the most controversial small churches in America.

Instead of turning America against gays and same-sex unions, they argue, Phelps may have instead helped to draw attention to the subject of religious intolerance and its impact on fellow Americans.

“We should give thanks for [Fred Phelps’] gift to American society,” writes Mark Silk, a religion professor Trinity College in Hartford, Conn. “So what’s the gift? It’s that he made religious hostility to homosexuality repulsive.

Mr. Silk’s comment comes amid a social media debate over whether those whom Phelps targeted during his nearly 60 years as leader of the church should themselves picket Phelps’s eventual funeral. Westboro became synonymous with the evangelical backlash to gay marriage, strands of which still persist as states and federal courts battle over state laws and amendments that ban gay marriage.

In late February, for instance, Arizona Gov. Jan Brewer vetoed a bill that would have given new protections to religious business owners to turn down gay customers, which critics saw as an invitation to discriminate against gay people.

In the past year, eight US states have legalized gay marriage, and the US Supreme Court has ruled that the federal government has to recognize same-sex marriages from states where the practice is legal. Last month, Kentucky’s attorney general made news when he said he would not appeal a federal court ruling striking down that state’s constitutional ban on gay marriages.

But while it’s impossible to pin down Westboro’s exact role in changing attitudes around gay marriage, what’s unmistakable is that support for gay marriage among religious Americans has gone up sharply over time and especially in the last year.

Support for gay marriage has gone up 11 percentage points since last year among black Protestants (to 43 percent), and up 7 points just in the last year (to 62 percent) among white mainline Protestants. Support, however, remains flat among Catholics (59 percent) and Evangelicals (23 percent), according to the most recent survey by the Pew Research Center.

Westboro came into full public light in 1998, when the group picketed the funeral of Mathew Shephard, a murdered gay man. At its high point, the 40-member church spent $250,000 a year traveling around the country to picket. The outrage they caused helped the group’s publicity campaign, as it received news coverage even for events they only threatened to picket but never did.

There remains a lot of pent-up anger about Phelps’s long career as a religious rabble-rouser.

Considered “hyper-Calvinists” by some, Westboro Baptist congregants have never actually tried to change anybody’s mind on the issue of gay marriage, says Mr. Bradley. Instead, their job has been to “spread and communicate [God’s] hatred and anger.”

Yet some lesbian, gay, bisexual, and transgender (LGBT) activists who long feared Phelps and his picketers say they came to see Phelps and his flock in a different light as time passed.

“People started to feel bad for the groups they were protesting, their tactics were actually backfiring,” writes Kristen Hotham Carroll, a contributor to The Huffington Post's "Gay Voices" blog. “They went from being a dreaded enemy of the LGBT community to almost a secret weapon. Their hate made us more sympathetic.”

As he led his largely family-clan church on picketing exploits around the country, eventually expanding to picket military funerals, Phelps cut a peculiar figure in American history.

In his early career, he was a civil rights lawyer lauded by the likes of the NAACP. Phelps's firm represented black Kansans in civil rights lawsuits against the city of Topeka, Kansas Power & Light, and Southwestern Bell, among other cases. But he was disbarred by the Kansas Supreme Court in 1979 for unethical behavior and was forced to surrender his license to practice in federal courts in 1989.

Even before Phelps entered hospice care, Westboro Baptist Church was in a period of transition. The church excommunicated Phelps last summer after he advocated “a kinder approach between church members,” his estranged son, Nate Phelps, told the Topeka Capital-Journal.